Lower severity of log trace for DB integrity error

LOG.exception is not appropriate, since a concurrent
port creation can occur and it has been dealt with
in the except block.

This avoids spooking users who are on the hunt for
real bugs.

Supports fix for 1239637

Change-Id: I9489702ee63db0b004fe41195fa4727af261d514
(cherry picked from commit 4c6906498a5ef0f11ada3ff2224e995f6b3062d8)
Merge "Lower severity of log trace for DB integrity error" into stable/havana